The invention lies in the field of seismic prospecting methods and apparatus. More particularly it concerns seismic apparatus employing very large numbers of separate recording channels, in which groups of S recording channels are collected into each of T array terminals (AT) spaced apart a distance D, where the geophone signals are amplified, digitized, and transmitted as trains of digital bits to the next AT.
Still more particularly, it concerns the design of a cable system that includes at least one digital conductor pair, and a plurality of S/2analog conductor pairs, fabricated into a plurality of identical cables D/2 in length, that can be connected to each other and to the array terminals. The cables are interchangeable end for end.
In the prior art, in seismic operations over the past 40 years, geophones on the ground have been connected to a recording unit by means of multi-conductor cables. Individual twisted pair conductors were connected from each geophone, to separate amplifiers in the recording unit. The number of conductor pairs in the cable always equalled the number S of geophone groups.
With very large numbers of channels, and very long cables, the cable system was sometimes made up of separate paralleled cables, so that the cables could be tapered in length. For example, there might be 4 separate cables, each with N/4 conductor pairs. One cable would be 1/4 of the total length (L/4) of the cable spread, and would handle the N/4 geophones closest to the recording unit. Another cable of length L/2 would handle the next N/4 geophones. Another cable 3L/4 long would handle the third group of N/4 geophones, and the fourth cable of length L would handle the last N/4 geophones.
In recent years, the seismic practice has come to use cables of many conductor pairs, made in short lengths, and connected to plugs at each end, so that the cables were identical, end for end. This required that the conductor pair connections would be the same at each plug. Thus, the economy of tapered cables could not be utilized, and the total number of conductor pairs needed is equal to N.
More recently (as described in U.S. Pat. No. 3,881,166) a system with a very large number of channels has been provided in which the total number of TS geophones are divided into T groups of S each. Each group of S geophones are connected by a cable with S conductor pairs into each of the T ATs, where the analog geophone signals are digitized, stored, and transmitted as trains of digital signals on a digital conductor pair to the next AT and so on.
Here again, the cables are broken into units of length D equal to the spacing between pairs of ATs. Each cable has takeouts to connect S geophones, D/S apart. Each cable can be turned end for end without effect. Each cable has S analog conductor pairs.